Magnetic position indicator

ABSTRACT

A new type of position indicator is disclosed herein for detecting the presence or absence of magnetic material which operates on the principle of either saturating or not saturating a transformer. The indicator comprises a modified E core having two magnetic flux paths with a portion thereof common to both flux paths. An AC excitation winding is wound in transformer relationship with a secondary detection winding and generates the flux in one of the magnetic paths. The flux in the second magnetic path is generated by a DC excitation winding, which provides a means of saturation control over the E transformer. The DC magnetic circuit is opened or closed by the absence or presence of the magnetic material being detected; the detection winding in the AC circuit being responsive to the corresponding change in flux.

[ June 26, 1973 ABSTRACT [6 Claims, 6 Drawing Figures PrimaryExaminerThomas J. Kozma Att0rney-A. T. Stratton and Z. L. Dermer A newtype of position indicator is disclosed herein for detecting thepresence or absence of magnetic material which operates on the principleof either saturating or not saturating a transformer. The indicatorcomprises a modified E core having two magnetic flux paths with aportion thereof common to both flux paths. An AC eit'citation winding iswound in transformer relationship with a secondary detection winding andgenerates the flux in one of the magnetic paths. The flux in the secondmagnetic path is generated by a DC excitation winding, which provides ameans of saturation control over the E transformer. The DC magneticcircuit is opened or closed by the absence or presence of the magneticmaterial being detected; the detection winding in the AC circuit beingresponsive to the corre sponding change in flux.

United States Patent 1 Santis et al.

lnventors: Dean C. Santis, Pittsburgh; Andre Wavre, Monroeville; AndrasI. Szabo, Export, all of Pa.

I73 I Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Nov. 27, I970 Appl. No.: 93,343

[52] U.S. 336/45, 336/131 [51] Int.

[58] Field of Search 336/30, 40, 45, 90,

[56] References Cited UNITED STATES PATENTS 2,395,88]Klemperer......................

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MAGNETIC MATERIAL MAGNETIC POSITION INDICATOR BACKGROUND OF THEINVENTION This invention pertains in general to a new magnetic positionindicator for detecting the presence or absence of magnetic material andis useful in detecting the presence or absence of control rods from thecore of a nuclear reactor.

One type of control rod drive mechanism for nuclear reactors is anelectrically controlled, hydraulically operated device whichindividually moves control rods between only two rest positions; eitherfull in or full out. Each control rod consists of at least one neutronabsorber element, approximately the same size as the fuel rods. Eachcontrol rod is connected to a hydraulic mechanism and travels in guidethimbles provided within the fuel assembly. In the inserted position,the absorber elements fit in the fuel assembly thimbles respectively andin the withdrawn position they move into guide tubes. This is the samearrangement used in current pressurized water reactor designs, exceptthat in the aforedescribed system the rods cannot assume anyintermediate positions other than being either fully inserted or fullywithdrawn. Each hydraulic control rod mechanism is a completelyindependent system and controls the movement of one drive rod which isconnected to the absorber rods associated with one control rod, in thisexample two interconnected absorber rods form one control rod. Eight ofthese independent mechanisms are located in a single control assembly,however, each mechanism must have its own valve coil and positionindicator coils to operate completely independent of the other seven.For a better understanding of the operation of the aforedescribedcontrol system reference may be had to application Ser. No. 700,153, nowUS. Pat. No. 3,519,535 filed Jan. 24, 1968 by Robert J. French et al.,entitled Nuclear Reactor and assigned to the Westinghouse ElectricCorporation.

Several types of indicators have been considered with respect to thiscontrol system for detecting the presence or absence of the control rodsfrom the nuclear core. The first indicator considered was a transformertype rod position indicator. This indicator consisted of a transformerwound on a C-core with an auxiliary coil in series with a secondarywinding. The poles of the C- core pass through the control rod housingand are welded to the housing. The magnetic circuit is opened or closedby the absence or presence of the magnetic drive rod which is used aspart of the magnetic circuit. When the drive rod is present and themagnetic circuit is closed, the secondary voltage is large due totransformer action, and when the rod is missing, the secondary voltageis small because of the large reluctance of the magnetic circuit andbecause of the auxiliary coil which helps to null out the secondaryvoltage. This scheme has two basic disadvantages, due to the shortingeffect of the non-magnetic metallic drive rod housing on thetransformer, which leads to its unreliable and ineffective use. First,it must be excited by a low frequency sinusoidal signal, for example Hz,which would require some sort of frequency converter as a source; andsecond, due to the shorting effect, the reliability of obtaining goodoutput signal magnitudes is directly affected by the effectiveresistance of the shorted turn which, as is well known, is nottemperature stable.

Another type of rod position indicator that was considered as a reedswitch type rod position indicator. This indicator consists of a reedswitch in series with a magnetic switch and a permanent magnet. The magnetic switch is actually the magnetic drive rod. When the drive rod ispresent, enough flux flows through the magnetic circuit to close thereed switch. When the rod is missing, the leakage flux that flows is notlarge enough to close the reed switch. The basic disadvantage of thistype of rod position indicator is its reliability. First, the reedswitch has a limited life which is an obvious disadvantage. Second, thereed switch rod position indicator is extremely sensitive to magneticinter ference and expensive shielding would be necessary.

Thus a new rod position" indicator was necessary to overcome thedisadvantages of those already considered. The desired rod positionindicator must have the following characteristics: It must be reliable;it must be temperature stable; it must be inexpensive; its use must notdestroy the sealed integrity of the mechanism housing; it must giveoutput signals which are large in mag nitude in order to eliminateamplification and noise problems; it must have good discriminationbetween the output signals that indicate the presence or the absence ofthe drive rod; it must not require expensive magnetic shielding; it mustnot slow down the scram time of the control rod; it must require aminimum amount of installation wiring; and it must be efficient to use.

SUMMARY OF THE INVENTION A new position indicator was devised toovercome the disadvantages of the prior art and to provide theaforementioned desired characteristics. This position indication systemuses two bistable position indicators per drive rod or movable elementto determine its position. The first indicator is located at oneextremity of movement of the movable element, i.e., at the top of themechanism, and is used to detect the presence of the drive rod in theupper portion of the drive rod housing. Similarly, the second indicatoris located at the other extremity of movement, i.e., at the bottom ofthe mechanism, and is used to detect the presence of the drive rod inthe lower portion of the housing. When the drive rod is in the full outposition, both the top and bottom indicators will detect its presence.When the rod is in the full in position, neither indicator will detectit. If something fails and the drive rod remains stuck in the middle,the top indicator will not detect it while the bottom indicator will. Amodified mechanism is also disclosed wherein the bottom indicator onlydetects the presence of a rod where the rod is neither full in nor fullout.

The basic principle of the indicator is that of a saturable transformer.The indicator comprises a transformer wound on a modified E core. Thepoles of the E core pass through the control rod housing and arehermetically secured to it as by welding. The indicator has a detectionwinding and a DC and AC excitation winding. The AC excitation windingand the detection winding are wound similar to primary and secondarytransformer windings. The DC winding provides a means of saturationcontrol over the E transformer. The DC excitation coil is driven from aDC current source which sets up a DC flux whose magnitude is dependenton the reluctance of the magnetic circuit. The primary of thetransformer, located on the center leg of the main core transformer isconnected to a detection unit, such as an oscilloscope.

The position indicator contains two magnetic circuits labelled A and Bfor convenience. These two magnetic circuits utilize common magneticpaths which are not orthogonal to each other. If the magnetic circuit Bis driven by a time varying magnetomotive force, the flux B, which willflow, is dependent on the reluctance of the magnetic circuit. Byincreasing the flux flowing in the magnetic circuit A, the reluctance ofthe common magnetic paths are increased; therefore, causing the flux Bto decrease. This change in the flux B can be detected by observing thevoltage drop across the secondary winding. The magnetic drive rodcompletes the magnetic circuit A. When the magnetic drive rod ismissing, the reluctance of magnetic circuit A is increased and thereforethe flux A associated therewith decreases. This in turn causes thereluctance of the common magnetic paths to decrease, therefore causingthe flux B and the voltage drop across the secondary to increase. Whenthe drive rod is present, the reluctance of magnetic circuit A isdecreased and therefore the flux A increases. This in turn causes thereluctance of the common magnetic path to increase, therefore causingthe flux B and the voltage drop across the secondary to decrease. Itshould be noted that this device utilizes the fact that magneticmaterial has a non-linear B-I-I relationship.

A new DC MMF biasing scheme is also provided herein. The new design wasnecessary to overcome an inherent condition in the hydraulic controlmechanism which makes it almost impossible to detect the presence of thecontrol rod drive shafts by the bottom indicators. The new biasingscheme comprises two primary coils which are wound around the outercircumference of the control rod housing. These coils are redundant andsupply the necessary DC MMF required to operate all of the indicators atthe bottom, eight indicators in this example. There is also a permanentmagnet embedded in one pole of each indicator. The magnets are used tonullify the DC leakage flux which flows then the rod is missing. Thephilosophy behind the use of the permanent magnets to nullify the DCleakage flux is that it is easier to detect a change of to 1 than achange of 100 to 101. This scheme in conjunction with the use of anon-magnetic section in the drive shaft, which is opposite the bottomindicator when the rod is in the full up condition, provides a workablesolution to the aforementioned rod detection problems.

DESCRIPTION OF THE DRAWINGS For a better understanding of an exemplaryembodiment of this invention, reference may be had to the accompanyingdrawings, in which:

FIG. 1 is a schematic diagram of a nuclear reactor and its control roddrive system embodying the novel position indicator of this invention;

FIGS. 2A and 28 when taken end-to-end represent an isometric view of thecontrol assembly of FIG. 1 with a portion thereof cut away for clarity,FIG. 2A showing the top portion of the assembly containing the topposition indicator and FIG. 23 showing the portion of the assemblycontaining the lower position indicator;

FIG. 3 is a schematic diagram of a rod position indicator, illustrativeof one embodiment of this invention;

FIG. 4 is a schematic diagram showing a crosssectional view of a controlassembly incorporating a modified DC biasing scheme for saturating thesaturable transformer indicator of FIG. 2; and

FIG. 5 is a schematic diagram showing a longitudinal sectional view ofthe control assembly of FIG. 4 taken along the lines V-V thereof,illustrating some novel modifications made to the circuit of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention is illustratedfor use with an electrically controlled, hydraulically operated,reactivity control system for the pressurized water reactors. As illus-"trated in FIG. 1, each control rod 42 consists of a pair tion. The pairof absorber elements 12 of each rod 42 V is connected to a hydraulicmechanism 16 and travels in guide thimbles, not shown, provided withinthe fuel assembly 20. In a typical design, about 400 control rods areprovided in a 193 fuel assembly core. A simplified drawing of the systemis shown in FIG. 1, which illustrates two identical assemblies 30 eachshowing a schematic view of one mechanism 10. The system includes theabsorber rods 12, the drive rods 14, the actuating mechanism 16, thepump 18 and the control system not shown.

In the inserted position, the absorber rods 12 fit in the fuel assemblythimbles, not shown, and in the withdrawn position they move into theguide tubes 24. This is the same arrangement used in the currentpressurized water reactor designs, except that in the system of FIG. 1the rods cannot assume intermediate positions; being either fullyinserted or fully withdrawn. The drive rods 14, each of which moves twoabsorber rods 12, are laterally supported above the guide tubes 24 andin the vessel head adapter 26, to provide added stability.

Each hydraulic control rod mechanism 10 is a completely independentsystem and controls the movement of one drive rod 14 which is connectedto two absorber rods 12. In this example, eight of theseindependentmechanisms are located in a single control assembly 30. Each drive rod14 is rigidly attached to a piston 32, which fits loosely in a cylinder34, above the core 36. The rod is lifted by momentarily opening thevalve 38, which allows liquid coolant 40, in the reactor vessel 41, toflow up the cylinder 34, lifting the piston 32, and thus the control rod42. The coolant then flows into a recirculation system 44, maintained ata fixed increment below the reactor cooling system pressure by acentrifugal pump 18. The pressure differential supplies the force forlifting only one piston 32 in a control rod at a time. Flow through eachmechanism 10 is controlled by a simple, solenoid-operated, normallyclosed valve 38, which discharges into a manifold 46, connected to thesuction side of the pump 18 as shown in FIG. 1. Upon withdrawal thepiston 32 is held in its upper position by an electromagnet 48 and thevalve 38 is closed. When the electromagnet 48 is deenergized, thecontrol rod 42 falls back into the core 36 by gravity.

In this example eight mechanisms are grouped together in a controlassembly and have a common fluid outlet. However, each mechanism has itsown valve 38, hold coil 48 and position indicator coils 56 and thusoperates completely independently of the other seven. The flow from eachcontrol assembly is collected in a piping network 50 and returned to aconnection on the reactor coolant piping 52. A single control assembly30, containing the eight individual control mechanisms 10, may beobserved in FIGS. 2A and 28. FIG. 2A illustrates the top portion of thecontrol assembly containing the top position indicator and FIG. 28illustrates the lower portion of the control assembly containing thelower position indicator. Each of the eight mechanisms 10, aresymmetrically arranged around the drive rod support column 54.

A new rod position indication system 56, referred to at times herein asthe saturable transformer rod position indication system, using magneticcircuits, has been developed for detecting the position of the controlrods 42, in the new control rod drive system of FIG. 1. The developmentwas necessary due to the fact that prior indicators were unreliable andinefficient to use. As mentioned above, in the FIG. 1 system, theindividual rods 42, have only two rest positions; either fully insertedor fully withdrawn. Unless a malfunction occurs, the individual rods 42,will never be in the middle.

This rod position indication system 56, uses two bistable rod positionindicators 58 and 60 per control rod 42, to determine its position. Thefirst indicator 58, is located at the top of the mechanism 10, and isused to detect the presence of the movable element or control rod 42, inthe upper portion of the housing 62. Similarly, the second indicator 60,is located at the bottom of the mechanism 10, and is used to detect thepresence of the control rod 42 in the lower portion of the housing 62.In one embodiment of this inventiomwhen the control rod 42, is in thefull out position, both the top indicator 58, and the bottom indicator60, will detect its presence. When the rod 42, is in the full insertedposition in the reactor core, neither will detect it. If failure occursand the control rod 42, remains stuck in the middle, the top indicator58, will not detect it while the bottom indicator 60 will. Such anindication system must have the following characteristics: it must bereliable; it must be temperature stable; it must be inexpensive and mustnot destroy the sealed integrity of the mechanism housing; it must giveoutput signals which are large in magnitude in order to eliminateamplification and noise problems; it must have good discriminationbetween the output signals that indicate the presence or the absence ofthe drive rod; it must not require expensive magnetic shielding; it mustnot slow down the scram time of the rod nor must it be capable ofholding the rod; it must require a minimum amount of installation wiringand it must be efficient to use.

An indication system which satisfies the aforementioned criteria isillustrated in FIG. 3. The magnetic core 65 of the indicator isgenerally C-shaped having two open legs 68 and a bight 63 connectedtherebetween. At least a portion of the bight forms a generally thetashaped main core 64. The DC excitation coil 66, is driven from a DCcurrent source, not shown, which sets up a DC flux duwhose magnitude isdependent on the reluctance of the magnetic circuit. The primary winding70, of the transformer, located on the center leg 74, of the main coreof the indicator, is driven from a sinusoidal current source 71 forexample a 60 Hz source. The secondary transformer winding 72, isconnected to a detection readout such as an oscilloscope, not shown.

The basic principle of operation is that of a saturable transformer. Theindicator includes a detection winding 72, and a DC and AC excitationwinding 66 and 70. The AC excitation winding and the detection winding72, are wound similar to primary and secondary transformer windings. TheDC winding 66, provides a means of saturation control over the E-coretransformer. In this embodiment the magnetic circuits A and B utilize,in part, common magnetic paths within the main core 64, which are notorthogonal to each other. The common magnetic paths or regions areillustrated in FIG. 3 by the reference characters C, D, E and F,respectively. The magnetic circuit B is driven by a time varyingmagnetomotive force imparted by winding 70. The flux 4),, which willflow in magnetic circuit B is dependent on the reluctance of themagnetic circuit. By increasing the flux flowing the magnetic circuit Athe reluctance of the common magnetic paths C, D, E and F are increased,therefore causing the flux da to decrease. This change in flux tb can bedetected by observing the voltage drop across the secondary coil 72, inmagnetic circuit B. When the rod 42, is missing, the reluctance of themagnetic circuit A is increased and therefore decreases. This in turncauses the reluctance of the magnetic path C, D, E and F to decrease;therefore, causing da and the voltage drop across the secondary coil 72,to increase. When the rod 42, is present, the reluctance of the magneticcircuit A is de creased and therefore (1), increases. This in turncauses the reluctance-of the magnetic paths C, D, E and F to increase;therefore, causing (1),, and the voltage drop across the secondary coil72, to decrease.

Referring now to FIGS. 4 and 5 it may be seen that a new arrangement,identified generally by the reference character 76 is described hereinto supply the DC MMF necessary to bias the saturable transformerindicator or the reed type of indicator used by the prior art. FIGS. 4and 5 are schematic representations of a control assembly 30, showingeach of the eight mechanisms 10, associated with each assembly 30 andthe lower indicators 60, associated with each mechanism 10. The instantarrangement for DC biasing is necessary to overcome an inherentcondition in the control mechanism which make it almost impossible todetect the presence of the control rod drive shafts 14, by the bottomindicators 60. One of the reasons why reliable detection was notpossible is that the change in DC flux from when the rod 14 is present,to when it is absent. is not large enough. The reason for this is thatwhen the drive shaft 14 is present at the bottom indicator 60, in theworst case, an air gap appeared between the shaft 14 and the magneticplugs 69, which is comparable to the air gaps between the adjacentshafts 14, and the plugs 69. Another reason is that the DC leakage fluxwhich flows through the indicator when the rod 14 is missing iscomparable to the flux which flowed when the rod 14 is present. Thefollowing changes were made in the control mechanism 10 to correct someof these difficulties: (1) The air gap 67 (shown in FIG. 3) between themagnetic plugs 69 and the poles 68 has been reduced; (2) the worst caseair gap between the magnetic plugs 69 and the control rod drive shaft14, has been reduced; and (3) a sandwich type of control rod drive shaft78, is used. The modified drive shaft 78 is so constructed that when therods 42 are in their full out position a non-magnetic portion such asthe insert 84 will appear in front of the indicators 60, at the bottom.If one adjacent rod 42 is struck, the indicators 60 on each side willnot detect its presence. If the two rods 42 adjacent to a givenindicator 60 are struck, the latter indicator 60 can read erroneously,since such a condition in a control assembly will call for immediateattention.

In accordance with the present invention, means are also provided forsupplying the DC MMF, as illustrated in FIGS. 4 and 5. Two coils 80 arewound around the outer circumference of the housing 62 and serve as theprimary redundant coils which supply the required DC MMF to operate alleight of the bottom indicators 60. There are also permanent magnetinserts 82 embedded in one pole 68 of each indicator 60. These permanentmagnets 82 are used to nullify the DC leakage flux which flows when therod 42 is missing. The philosophy behind the use of the permanentmagnets 82 for nullifying the DC leakage flux is that it is easier todetect a change of to 1 than a change of 100 to l0l. This scheme inconjunction with the use of a non-magnetic insert 84 in the drive shaft14, which is positioned opposite the bottom indicator 60 when the rod 42is in the full up condition, provides a workable solution to the problemof rod detection. The permanent magnets 82 are not necessary in the topindicators 58 (shown in FIGS. 1 and 2) for satisfactory operation, evenwhen two adjacent drive shafts 14 are present. This is due to thedifference in cross section of the control mechanism of the two pointsas may be observed in FIGS. 2A and 2B. Though it should be understoodthat the new indicator design may be used for both the top and bottomindicators with considerable advantage. Thus, the indication system 56of this invention has three important advantages over previous designs.First and most important it provides an acceptable solution to theproblem of rod detection. Second through the use of redundancy, the lossof one excitation coil 80 does not affect the operation of theindicators 60, whereas the loss of a coil in the indicators of the priorart would mean the loss of at least one indicator. Third the use of thenew scheme has an economic advantage over the old scheme, inasmuch asseparate DC biasing coils are not needed for each indicato'nln theillustrated embodiment two DC coils are used to bias eight indicatorsthus cutting the total number of wires needed from 64 to 40 perassembly.

There are many modifications that may be made to the aforementionedcircuitry to maintain specific design criteria, without departing fromthe scope of this invention. Such modifications may be used to overcomeproblems involving: Magnetic interference; a reduction in holding forceson the control rods; increased magnitude of the output voltage; improvedoutput voltage ratios; and temperature stabilization. The resultantdesign changes will depend upon the environment in which the inventionis used.

Two modifications are available for minimizing the affects of magneticinterference on the saturable transformer rod position indicator. Thefirst is to make the flux (12,, large in magnitude compared to theleakage fl-uxes which may be coupled in. This can be done by increasingthe cross-section area of the main core 64 (illustrated in FIG. 3) whichin turn allows the flux (1),, to be increased. Care must be taken sothat the pole pieces 68 do not saturate before the main core 64saturates in regions C, D, E and F. The second is to add some magneticmaterial so as to divert the leakage fluxes away from the main core 64of the indicator. This arrangement is utilized in the preferredembodiment illustrated above in FIGS. 4 and 5.

There are four ways to reduce the holding forces of the indicator on thedrive rod 14. These four modifica' tions relate to methods of decreasingthe flux which flows through the pole pieces 68. The first consists ofdecreasing the cross-section area of the main core 64 in the regions C,D, E and F, which results in the need of less flux to control thereluctance of these regions. The second and third respectively comprisedecreasing the DC excitation current and decreasing the number of turnsof the DC excitation coil 66. In order to maintain a reasonable outputsignal, when these methods are used, it may be necessary to decrease thecrosssection area of the main core 64 as described above. The fourthmethod consists of increasing the reluctance of the pole pieces 68 whichis achieved by adding air gaps between the pole piece and the magneticplugs as shown in FIG. 3 by reference character 67.

There are two alternatives which may be employed to increase themagnitude of the output voltages. The first consists of adding more ironto the main core 64 which in turn will increase the change in flux. Thesecond consists of increasing the number of turns and turns ratio of thetransformer.

There are two methods for improving the output voltage ratios. The firstconsists of selecting the DC excitation current which gives a maximumratio. The second consists of reducing the leakage flux coupling betweenthe primary and secondary coils when the main core 64 is saturated. Thiscan be done by leaving the primary coil 70 on the center leg 74 of themain core 64 and splitting the secondary coil 72 up among the regions C,D, E and F of the main core.

In addition there are two alternatives which may be employed tostabilize the indicator with temperature change. The first is to selectmagnetic material for use in the indicator whose magnitudecharacteristics are stable with temperature, such as 410 stainlesssteel. The second has already been illustrated in the aforementionedpreferred embodiment. It involves using current sources for excitation;thereby eliminating the effects due to the resistance changes of thecoils with temperature.

It is to be understood that these modifications are merely illustrativeof some other embodiments of this invention and are not meant to belimitative thereof.

We claim as our invention:

1. A magnetic position indication system for detecting the presence orabsence of a movable magnetic member having at least one magneticposition indicator comprising a magnetic core, said magnetic core havinga first and a second magnetic loop and a magnetic flux path common to atleast a portion of said first and second magnetic loops, the first ofsaid magnetic loops forming a closed magnetic flux path and having ACflux excitation means coupled thereto, the second of said magnetic loopsforming a normally opened circuited magnetic flux path and having DCflux excitation means coupled thereto, a movable magnetic member beingmovable to close the circuit of said second magnetic loop, whereby thepresence of said movable member substantially between the open circuitof said second magnetic loop closes the DC magnetic flux path associatedtherewith to enable the DC flux flowing therein to saturate the magneticcore and thereby increase the reluctance of said common path, detectionmeans in said first magnetic loop responsive to a change in the AC fluxflowing therein due to the increased reluctance of said common magneticpath.

2. The magnetic position indicator of claim 1 wherein said detectionmeans is placed in transformer relationship with said AC flux excitationmeans.

3. The magnetic position indicator of claim 1 wherein said AC'fluxexcitation means comprises an AC coil magnetically coupled to said firstmagnetic loop.

4. The magnetic position indicator of claim 1 wherein said magnetic corecomprises a generally C-shaped core having two open legs and a bightconnected there between, said open legs being mounted on said housingand at least a portion of said bight being generally theta shaped.

5. The magnetic position indicator of claim 1 including at least onepermanent magnet coupled to at least one loop of said magnetic core.

6. The magnetic position indication system of claim 1 including aplurality of said magnetic position indicators, and at least a portionof said magnetic position indicators having a common DC flux excitationmeans comprising at least one winding at least partially enclosing themagnetic cores of each position indicator of said portion of positionindicators.

7. The magnetic position indicator of claim 1 wherein said movablemember is so constructed that in at least one of its positionsrelatively non-magnetic material substantially bridges the open circuitof said second magnetic loop and in its remaining positions magneticmaterial substantially bridges the open circuit of said second magneticloop.

8. The magnetic position indicator of claim 1 wherein said movablemember comprises a portion of nuclear reactor control rod drive shaft.

9. The magnetic position indication system of claim 8 wherein saidmagnetic position indicator is mounted on a nuclear reactor control rodhousing having said control rod drive shaft slidably mounted therein.

10. The magnetic position indication system of claim 9 including twomagnetic position indicators longitudinally spaced along said housingfor determining the presence or absence of said control rod shaft atsaid indicator location.

11. The magnetic position indication system of claim 10 wherein saidcontrol rod shaft is longitudinally movable to an upper and a lower restposition and said magnetic position indicators are substantially locatedat the upper extremity of said upper rest position and slightly abovethe upper extremity of said lower rest position respectively.

12. The magnetic position indication system of claim 9 wherein aplurality of said control rod shafts are circumferentially positionedaround the inner wall of said housing and a plurality of said magneticposition indicators are circumferentially positioned around said housingadjacent said control rod shafts respectively.

13. The magnetic position indication system of claim 12 wherein each ofsaid control rod shafts has a first and a second indicator respectively,said control rod shafts being longitudinally movable to an upper and alower rest position and said first and second indicators aresubstantially positioned at the upper extremity of said upper restposition and slightly above the upper extremity of said lower restposition respectively.

14. The magnetic position indication system of claim 13 wherein saidsecond indicators have a common DC flux excitation means coupledthereto, said DC excitation means comprising at least one windingpartially enclosing the magnetic cores of each of said secondindicators.

15. The magnetic position indication system of claim 9 wherein saidmagnetic core comprises a generally C- shaped core having two open legsand a bight connected therebetween, said open legs being mounted on saidhousing and at least a portion of said bight being generally thetashaped.

16. A magnetic position indication system for detecting the presence orabsence of magnetic material having a plurality of magnetic positionindication means, at least a portion of said magnetic positionindication means having a DC flux path and a DC flux excitation meansassociated therewith, said DC flux excitation means being common to atleast two of said indication means, comprising at least one winding atleast partially enclosing the DC flux paths of said two indicationmeans.

* l R t

1. A magnetic position indication system for detecting the presence orabsence of a movable magnetic member having at least one magneticposition indicator comprising a magnetic core, said magnetic core havinga first and a second magnetic loop and a magnetic flux path common to atleast a portion of said first and second magnetic loops, the first ofsaid magnetic loops forming a closed magnetic flux path and having ACflux excitation means coupled thereto, the second of said magnetic loopsforming a normally opened circuited magnetic flux path and having DCflux excitation means coupled thereto, a movable magnetic member beingmovable to close the circuit of said second magnetic loop, whereby thepresence of said movable member substantially between the open circuitof said second magnetic loop closes the DC magnetic flux path associatedtherewith to enable the DC flux flowing therein to saturate the magneticcore and thereby increase the reluctance of said common path, detectionmeans in said first magnetic loop responsive to a change in the AC fluxflowing therein due to the increased reluctance of said common magneticpath.
 2. The magnetic position indicator of claim 1 wherein saiddetection means is placed in transformer relationship with said AC fluxexcitation means.
 3. The magnetic position indicator of claim 1 whereinsaid AC flux excitation means comprises an AC coil magnetically coupledto said first magnetic loop.
 4. The magnetic position indicator of claim1 wherein said magnetic core comprises a generally C-shaped core havingtwo open legs and a bight connected therebetween, said open legs beingmounted on said housing and at least a portion of said bight beinggenerally theta shaped.
 5. The magnetic position indicator of claim 1including at least one permanent magnet coupled to at least one loop ofsaid magnetic core.
 6. The magnetic position indication system of claim1 including a plurality of said magnetic position indicators, and atleast a portion of said magnetic position indicators having a common DCflux excitation means comprising at least one winding at least partiallyenclosing the magnetic cores of each position indicator of said portionof position indicators.
 7. The magnetic position indicator of claim 1wherein said movable member is so constructed that in at least one ofits positions relatively non-magnetic material substantially bridges theopen circuit of said second magnetic loop and in its remaining positionsmagnetic material substantially bridges the open circuit of said secondmagnetic loop.
 8. The magnetic position indicator of claim 1 whereinsaid movable member comprises a portion of nuclear reactor control roddrive shaft.
 9. The magnetic position indication system of claim 8wherein said magneTic position indicator is mounted on a nuclear reactorcontrol rod housing having said control rod drive shaft slidably mountedtherein.
 10. The magnetic position indication system of claim 9including two magnetic position indicators longitudinally spaced alongsaid housing for determining the presence or absence of said control rodshaft at said indicator location.
 11. The magnetic position indicationsystem of claim 10 wherein said control rod shaft is longitudinallymovable to an upper and a lower rest position and said magnetic positionindicators are substantially located at the upper extremity of saidupper rest position and slightly above the upper extremity of said lowerrest position respectively.
 12. The magnetic position indication systemof claim 9 wherein a plurality of said control rod shafts arecircumferentially positioned around the inner wall of said housing and aplurality of said magnetic position indicators are circumferentiallypositioned around said housing adjacent said control rod shaftsrespectively.
 13. The magnetic position indication system of claim 12wherein each of said control rod shafts has a first and a secondindicator respectively, said control rod shafts being longitudinallymovable to an upper and a lower rest position and said first and secondindicators are substantially positioned at the upper extremity of saidupper rest position and slightly above the upper extremity of said lowerrest position respectively.
 14. The magnetic position indication systemof claim 13 wherein said second indicators have a common DC fluxexcitation means coupled thereto, said DC excitation means comprising atleast one winding partially enclosing the magnetic cores of each of saidsecond indicators.
 15. The magnetic position indication system of claim9 wherein said magnetic core comprises a generally C-shaped core havingtwo open legs and a bight connected therebetween, said open legs beingmounted on said housing and at least a portion of said bight beinggenerally theta shaped.
 16. A magnetic position indication system fordetecting the presence or absence of magnetic material having aplurality of magnetic position indication means, at least a portion ofsaid magnetic position indication means having a DC flux path and a DCflux excitation means associated therewith, said DC flux excitationmeans being common to at least two of said indication means, comprisingat least one winding at least partially enclosing the DC flux paths ofsaid two indication means.